This invention relates to methods, apparatuses and systems for isolating vibrations emanating from sources such as machinery, more particularly to those which implement a resilient element and which provide support for such sources.
It is environmentally desirable in many contexts to reduce transmission of vibrations to neighboring structure. For example, the U.S. Navy has an interest in attenuating the transmission, via connecting members to supporting structure, of unwanted vibrations from heavy machinery such as ship engines. Devices for reducing such transmission are generally known as vibration "isolators" because they serve to "isolate" the machine's vibration from contiguous structure. A vibration isolator is used to join one object to another and to restrict, to some degree, the transmission of vibration. Both passive and active vibration isolation systems have been known in the art.
Passive vibration isolators have conventionally involved a passive damping arrangement which provides a resilient element ("spring") along with a damping mechanism ("energy releaser"), and which serves as a support ("mount"), for vibrating machinery or other structure. Passive vibration isolation devices, alternatively referred to as "mounts" or "springs" or "spring mounts" in nomenclature, operate on the principle of low dynamic load transmissibility by a material having a resiliant property. Passive mounts are designated "passive" because their function is based upon their inherent property rather than on their ability to, in an "active" manner, react to an in-situ condition.
Passive mounts have been known to use any of various materials for the resilient element, such as rubber, plastic, metal and air. Elastomeric mounts rely primarily upon the resilience and the damping properties of rubber-like material for isolating vibrations. Mechanical spring mounts implement a helical or other metal spring configuration. Pneumatic mounts utlilize gas and an elastic material (such as reinforced rubber) as resilient elements in a bellows-like pneumatic spring assembly. A pneumatic mount or spring typically comprises a flexible member, which allows for motion, and a sealed pressure container or vessel having one or more compartments, which provides for filling and releasing a gas.
Pneumatic springs are conventionally referred to as "air springs" because the gas is usually air. In conventional usage and as used herein the terms "air spring," "air mount" and "air spring mount" are used interchangeably, and in the context of these terms the word "air" means "gas" or "pneumatic," wherein "gas" or "pneumatic" refers to any gaseous substance.
In general comparison with other material-type passive mounts, air mounts are advantageous by virtue of their lighter weight, greater energy-storage capacity per unit weight, and impedance tuning capability by means of air pressure adjustment. Due to their greater resilience, air mounts typically have lower resonant frequencies than have other passive mount types. Consequently, air mounts are, in general, more effective vibration isolators.
One important factor, however, for an air mount to be as effective as designed, and which has often been neglected, is the rigidity of the backing or the rigidity of the below-mount foundation. The stiffer the foundation, the better the performance of the isolation mount. An ideal foundation is a rigid base; however, for most passive vibration isolation applications, design and effectuation of a below-mount structure approximating a rigid base is impractical or cost-ineffective. Suffice it to say that there remains appreciable room for improvement in the majority of passive vibration isolation applications. The need is extant for a more effective yet feasible and economical approach to passive vibration isolation.
Active vibration isolation has more recently become known in the art. Basically, a sensor measures the structure's vibration, an actuator is coupled with the structure, and a feedback loop tends to reduce the unwanted motion. Typically, an output signal, proportional to a measurable motion (such as acceleration) of the structure, is produced by the sensor. Generally speaking, the actuator includes some type of reaction mass. A processor/controller processes the sensor-generated output signal so as to produce a control signal which drives the reaction mass, the actuator thereby producing a vibratory force, whereby the motion (e.g., acceleration) of the structure is reduced.
The three basic components of an active vibration isolation system are a motion sensor (e.g., a motion transducer), a processor/controller and a vibratory actuator. The sensor responds to vibratory motion by converting the vibratory motion into an electrical output signal that is functionally related to, e.g., proportional to, a parameter (e.g., displacement, velocity or acceleration) of the experienced motion. An accelerometer, for example, is a type of sensor wherein the output is a function of the acceleration input; the output is typically expressed in terms of voltage per unit of acceleration. The most common processor/controller is a "proportional-integral-derivative"-type ("PID"-type) controller, a kind of servomechanism, which proportionally scales, and integrates or differentiates, the sensor response. The actuator is essentially a device adapted to transmitting a vibratory force to a structure; such an actuator has been variously known and manifested as an inertia actuator, inertial actuator, proof mass actuator, shaker, vibration exciter and vibration generator; as used herein, the terms "actuator," "inertia actuator" and "vibratory actuator" are interchangeable and refer to any of these devices. The actuator generates a force, applied to the structure, based on the electrical output signal from the processor/controller.